Quartzelec repairs Ghanaian power plant rotor on-time, on-budget 23 May 2013

Ghanaian power producer TAQA is reporting that UK-based electrical engineering group Quartzelec rose successfully to the challenge recently in refurbishing a failed generator rotor within six months.

The rotor is part of a T240-370, 154MVA generator at the Takoradi thermal power plant, in Ghana, which comprises two GE Frame 9E combustion gas turbines. It had suffered a double earth fault, resulting in severe arcing damage.

It measures eight metres in length and weighs 36.5 tonnes, and was substituted with a new spare rotor purchased from GE to allow the generator to continue operating.

Steve Cooper, business development for power generation at Quartzelec, explains that the arcing damage had spread to every slot along the wedge dovetails and on the retaining rings of the rotor, rendering the machine unusable.

Two deep excavations – one 250mm and the other 300mm axially – were found within two of the forging slots, where material had vaporised. In addition, more than 10% of the copper winding on the rotor had either been melted away or was distorted so unrecoverable.

"With a replacement generator costing in excess of £2 million, the client was looking for a technically competent yet commercially viable solution to re-establish full generating capacity," states Cooper.

"An initial damage assessment had been undertaken by the OEM but we were awarded the contract, based on our technical expertise and experience in electrical repair and maintenance," he adds.

The rotor components were transported to Quartzelec's newly commissioned Rugby facility, where omplex weld repair was performed on the two damaged slots to salvage the forging.

A series of replication tests were then conducted before arcing damage along the slot dovetail was machined off – increasing the dovetail dimensions – and a new set of retaining rings, snap rings and wider wedges were procured.

The rotor was then rewound, using about 80% of the existing copper and a new design of insulation system. New end-winding packing blocks were also designed, manufactured and fitted.

In order to confirm overall mechanical integrity of the forging and new components, a set of NDT (non destructive testing) tests and a 120% over-speed test were performed, followed by a dynamic set of final electrical and mechanical tests before the rotor was suitably packed and shipped back.

"I really had no doubt on the success of the final test," comments Kirk Butler, director, O&M, Power & Water for TAQA. "This was a very important project for both Takoradi and TAQA and we are very pleased with the outcome."

TAQA has since awarded another contract to Quartzelec – involving refurbishment of a failed TKJ86-16 brushless exciter.

Brian Tinham

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